A fuel cell is a system for producing electric power. In a fuel cell, chemical reaction energy between oxygen and hydrogen contained in hydrocarbon-group materials such as methanol, ethanol, and natural gas is directly converted into electric energy.
Depending on the type of electrolyte used in the fuel cell, the fuel cell is classified into different types, including: phosphate fuel cell, molten carbonate fuel cell, solid oxide fuel cell, and polymer electrolyte or alkali fuel cell. Although each of these different types of fuel cells operate using the same principles, they differ in the type of fuel, catalyst, and electrolyte used, as well as in drive temperature.
A polymer electrolyte membrane fuel cell (PEMFC) has been developed recently. As compared to other fuel cells, the PEMFC has excellent output characteristics, a low operating temperature, and fast starting and response characteristics. The PEMFC may be used in vehicles, in the home and in buildings, and for the power source in electronic devices. The PEMFC, therefore, has a wide range of applications.
The basic components of the PEMFC are a stack, reformer, fuel tank, and fuel pump. The stack forms the main body of the fuel cell. The fuel pump supplies fuel in the fuel tank to the reformer. The reformer reforms the fuel to create hydrogen gas, and supplies the hydrogen gas to the stack. Accordingly, the PEMFC sends the fuel in the fuel tank to the reformer by operation of the fuel pump. The fuel is reformed in the reformer to generate hydrogen gas, and the hydrogen gas is chemically reacted with oxygen in the stack to thereby generate electric energy.
In the fuel cell system described above, the reformer is a device that reforms liquid fuel by a chemical catalytic reaction to generate hydrogen gas. The reformer also reduces a concentration of harmful materials such as carbon monoxide that deteriorate the fuel cell to cut down its service life. The reformer includes a reforming portion for generating hydrogen gas and a removing portion to remove the carbon monoxide contained with the hydrogen gas. The reforming portion generates hydrogen-rich gas from liquid fuel by the catalytic reaction of a steam reform, partial oxidation, and/or natural reaction. To reduce the concentration of carbon monoxide contained in the hydrogen gas, the removing portion uses a catalytic reaction such as a water gas conversion method, a selective oxidation method, and/or a method of refining hydrogen using a separating layer.
In the fuel cell system described above, the reforming portion is provided with an exothermic reaction portion which introduces a catalytic oxidation reaction of the fuel and air to generate thermal combustion, and an endothermic reaction portion which introduces a catalytic reforming reaction with the thermal combustion of the exothermic reaction portion to generate the hydrogen-rich gas. The endothermic reaction portion uses a predetermined thermal reaction from the exothermic reaction portion so that it promotes a catalytic reforming reaction of the fuel mixture mixed with liquid fuel and water to convert it into the hydrogen-rich gas. Thus, such a reformer requires an adiabatic portion for protecting the thermal energy generated in the reforming portion from leakage in order to increase a product reaction efficiency of the hydrogen gas.
However, in the conventional fuel cell system, the adiabatic portion is constructed as a singular layer contacting the surface of the reforming portion. Accordingly, there is a problem in that a high heat transfer quantity is discharged from the surface of the reforming portion because the heat transfer path passing through the adiabatic portion is short. Further, in the conventional fuel cell system, the reformer is repeatedly operated such that a heat residence stress occurs on the surface of the reforming portion and concentrates locally thereon. Therefore, there is another problem in that the joining performance of the adiabatic portion and the reforming portion become degraded.